The semiconductor integrated circuit industry has experienced rapid growth in the past several decades. Technological advances in semiconductor materials and design have produced increasingly smaller and more complex circuits. These material and design advances have been made possible as the technologies related to processing and manufacturing have also undergone technical advances. In the course of semiconductor evolution, the number of interconnected devices per unit of area has increased as the size of the smallest component that can be reliably created has decreased.
Semiconductor fabrication relies heavily on the process of photolithography, in which light of a given frequency is used to transfer a desired pattern onto a wafer undergoing semiconductor processing. To transfer the pattern onto the wafer, a photomask (also referred to as a mask or reticle) is often used. The photomask permits and prevents light in a desired pattern onto a layer of the wafer, such as a photoresist (PR) layer, which chemically reacts to the light exposure, removing some portions of the PR and leaving other portions. The remaining PR is then used to pattern an underlying layer. As feature sizes have decreased, the wavelength of light used in photolithography to pattern layers has decreased as well, creating additional difficulties and necessitating technological advances such as the use of extreme ultraviolet (EUV) as a light source and also phase-shifting masks. Improving photomasks is important to the continued advances in the industry, because various kinds of imperfections or problems in the resulting patterned layer can be compounded during subsequent processing steps as semiconductor device or integrated circuit is made. Photomask improvements include improvements made to the mask blank on which the photomask is derived and patterning the photomask.
For example, during phase-shifting mask fabrication, defects such as thin semi-transparent residue can be formed which often need localized etching to repair. The localized etching process can result in damage to an underlying mask substrate because a proper end point of the etch may be difficult to detect. Therefore, while some current techniques for creating photomasks have been adequate, they have not been entirely satisfactory in every aspect.